Generally, the natural frequency of the blades in the tangential direction is the lowest in the various vibration modes and correspondingly has the greatest effect on the moving blades. Therefore, it is desirable to restrain or dampen the tangential vibration of the moving blades.
Apparatus is known to restrain tangential vibration by tying the moving blades together, but members are disposed between the blades and welded to the blades. Such apparatus has the danger that the moving blades will crack at the welding portions due to the vibration of the moving blades.
Other apparatus is also known wherein a sleeve is inserted into a boss provided on each of the opposite surfaces of the blades, so that surface friction applied between the sleeve and the boss by centrifugal force of the sleeve due to rotation of the sleeve dampens the vibration of the moving blades. In this apparatus, there is not enough surface friction to satisfactorily dampen the vibration. Further, the apparatus will prevent the free twisting movement between the sleeve and the boss, because the boss moves in a direction traversing insertion of the boss into the sleeve with the moving blade itself rotated to produce centrifugal force.